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Dengue vector control strategies in an urban setting: an economic modelling assessment

Dr Paula Mendes, et al.

Background – An estimated 2·5 billion people are at risk of dengue. Incidence of dengue is especially high in resource-constrained countries, where control relies mainly on insecticides targeted at larval or adult mosquitoes. We did epidemiological and economic assessments of different vector control strategies.

Methods – We developed a dynamic model of dengue transmission that assesses the evolution of insecticide resistance and immunity in the human population, thus allowing for long-term evolutionary and immunological effects of decreased dengue transmission. We measured the dengue health burden in terms of disability-adjusted life-years (DALYs) lost. We did a cost-effectiveness analysis of 43 insecticide-based vector control strategies, including strategies targeted at adult and larval stages, at varying efficacies (high-efficacy [90% mortality], medium-efficacy [60% mortality], and low-efficacy [30% mortality]) and yearly application frequencies (one to six applications). To assess the effect of parameter uncertainty on the results, we did a probabilistic sensitivity analysis and a threshold analysis.

Findings – All interventions caused the emergence of insecticide resistance, which, with the loss of herd immunity, will increase the magnitude of future dengue epidemics. In our model, one or more applications of high-efficacy larval control reduced dengue burden for up to 2 years, whereas three or more applications of adult vector control reduced dengue burden for up to 4 years. The incremental cost-effectiveness ratios of the strategies for two high-efficacy adult vector control applications per year was US$615 per DALY saved and for six high-efficacy adult vector control applications per year was $1267 per DALY saved. Sensitivity analysis showed that if the cost of adult control was more than 8·2 times the cost of larval control then all strategies based on adult control became dominated.

Interpretation – Six high-efficacy adult vector control applications per year has a cost-effectiveness ratio that will probably meet WHO’s standard for a cost-effective or very cost-effective intervention. Year-round larval control can be counterproductive, exacerbating epidemics in later years because of evolution of insecticide resistance and loss of herd immunity. We suggest the reassessment of vector control policies that are based on larval control only.

Scientists have developed an improved method for controlling container-inhabiting mosquitoes responsible for the transmission of chikungunya and dengue viruses.

New pellet formulations of a chemical that kills mosquito larvae and pupae — Agnique — were used for the first time in combination with another insecticide — Altosid — in laboratory and field tests. Combined, the two insecticides had a longer-lasting effect than using either one alone.

They were used in a spray to coat water-filled containers where mosquitoes deposit their eggs.

In the laboratory, this provided 80 per cent mosquito control for 60 days, and 95 per cent control for at least 32 days in the field trials, according to the study published in this month’s (May) issue of The American Journal of Tropical Medicine and Hygiene.

The spray’s dual action interfered with mosquitoes’ development as well as the surface tension of the water — preventing larvae and pupae from remaining on the surface to breathe.

Altosid also remained effective after 107 days of a drought simulated in the laboratory, suggesting that it could prevent mosquitoes breeding even when dried-out water containers are refilled after periods of drought.

Co-author of the study Banugopan Kesavaraju, entomologist at US-based Salt Lake City Mosquito Abatement District, told SciDev.Net that “the container mosquitoes like Asian tiger mosquitoes, Aedes albopictus, colonise containers that can hold small quantities of water such as trash cans, cemetery vases and toys, which are more common in urban and suburban areas”.

He said that this makes the use of pesticides difficult in urban areas. Since the combined spray targets all mosquito life stages in the containers, this would permit better long-term mosquito control, he added.

Nuananong Jirakanjanakit, biomedical researcher at Mahidol University, Thailand, said this new method could be a realistic way of tackling mosquitoes and capping the transmission of diseases such as chikungunya and dengue fever.

Chikungunya virus is spread by mosquitoes found in standing water in urban areas. It re-emerged on several islands in the Indian Ocean in 2005, caused an outbreak in India in 2006–07, and was reported in Europe in 2007.

Dengue is now the most rapidly spreading mosquito-borne viral disease in the world (see also Dengue fever surges in Americas). In the absence of an effective drug or vaccine, vector control is the only way to reduce disease transmission.

Theeraphap Chareonviriyaphap, entomologist at Thailand-based Kasetsart University, welcomed the new development as “a major breakthrough in the battle against dengue and chikungunya viruses”. He told Scidev.Net that this is “a very simple combination but with a powerful effect on the ground leading to the killing of most vectors hidden in the containers”.

But Michael Turell, medical entomologist at USArmy Medical Research Institute of Infectious Diseases told SciDev.Net that educating people about how discarded trash provides breeding ground for disease-carrying mosquitoes could be as important as improving insecticides. “If these [trash cans] were turned upside down so that they would not hold water, I think it would have an even greater effect than a more efficient formulation of insecticides.”

Objective – To study dengue vector breeding patterns under a variety of conditions in public and private spaces; to explore the ecological, biological and social (eco-bio-social) factors involved in vector breeding and viral transmission, and to define the main implications for vector control.

Methods – In each of six Asian cities or periurban areas, a team randomly selected urban clusters for conducting standardized household surveys, neighbourhood background surveys and entomological surveys. They collected information on vector breeding sites, people’s knowledge, attitudes and practices surrounding dengue, and the characteristics of the study areas. All premises were inspected; larval indices were used to quantify vector breeding sites, and pupal counts were used to identify productive water container types and as a proxy measure for adult vector abundance.

Findings – The most productive vector breeding sites were outdoor water containers, particularly if uncovered, beneath shrubbery and unused for at least one week. Peridomestic and intradomestic areas were much more important for pupal production than commercial and public spaces other than schools and religious facilities. A complex but non-significant association was found between water supply and pupal counts, and lack of waste disposal services was associated with higher vector abundance in only one site. Greater knowledge about dengue and its transmission was associated with lower mosquito breeding and production. Vector control measures (mainly larviciding in one site) substantially reduced larval and pupal counts and “pushed” mosquito breeding to alternative containers.

Conclusion – Vector breeding and the production of adult Aedes aegypti are influenced by a complex interplay of factors. Thus, to achieve effective vector management, a public health response beyond routine larviciding or focal spraying is essential.

Since 1994, a strange and quiet mosquito has bred in Taiz. This kind of mosquito transmits dengue fever [virus] among people so that the pandemic increased in the city as the mosquito became established.

Despite differences over the number of infections with the disease and fatal cases, all agree that the disease has become established in Taiz and plagued the city amid ignorance of concerned bodies despite of the fact that its danger is increasing. “If concerned bodies would have intervened early, the matter would have been easier. We ignore the disease, its reasons, and the environment in which it reproduces.

We do not even know the protection against the disease because we didn’t know it was dengue fever [until] only lately,” said a Taizi citizen. Physician Samir Sufian, director of Al-Rawdha Hospital, reveals the size of the catastrophe affirming that public and private hospitals receive cases of dengue fever. “Like any other public and private hospital, we perform our duty and receive many cases, whether infected with dengue fever [virus] or other cases suffering the same
complications of the disease,” he said. “We have been receiving more than 100 cases a day, all of them are not necessarily infected with the disease [virus] but there are similar cases.

Only 72 cases have been hospitalized and [we] gave them blood platelets as the disease reduces the blood platelets to under 60 000 units. All these cases have recovered,” he said.

Concerning fatalities, the director of Al-Rawdha Hospital indicates that fatal cases have been occurring with the spread of the disease but indicates “death could happen if there are accompanying diseases.” He estimated the number of mortalities has reached only 7 and there are similar fatal cases in other hospitals. ” All of us are responsible for society with all [its health] categories, the health office is not solely responsible for deterring the disease and it is not our responsibility to exaggerate the number of infections and fatal cases,” said Sufian.

However, the technical director of Al-Safwa Hospital, Abdurrahman Saeed, affirms that the hospital has received hundreds cases during the past 2 months, but he indicates that [dengue] infections with disease have become rare during the past month. He expected that fatal cases in Taiz are estimated in tens. “In our hospital, for example, 3 fatal cases have been registered.”

The secretary general of health, Abdul-Jalil Azuraiqi, says, “The disease was not new, if concerned bodies had intervened early in 1994, the situation would have been better. The disease is now terribly rampant and there are not enough efforts for combating it.”

Deputy minister of Public Health and Population for Health and Planning Development, Jamal Nasher, says the ministry is working through its office in Taiz to limit the pandemic by curing [treating] all cases and helping the office to find diagnostic means to recognize dengue fever.

According to him, the strategy aims at eradicating the environment where the mosquito spreads the dengue fever via eliminating the factors [that] led to the mosquito establishment in the city. These factors are dominated by personal behavior, like storing water for a long time due to the lack of water in the city, and waste from workshops, houses, and cars.

Based on these factors, the strategy is divided into 2 parts; the 1st one concerns solving the water crisis while the 2nd is to clean the city of all trash to get rid of [breeding sites] of the mosquito which spreads the disease [virus] in
addition to awareness campaigns on the dangers of the disease and [for individuals,] how to protect themselves against it.

Director of Taiz Health Office, Abdul-Naser al-Kabab, says the disease was established in the city and the water crisis is the main reason. He said the 1st appearance of the disease in Taiz city was in 1994 with one confirmed case and another case found in Jeddah, Kingdom of Saudi Arabia at that time. The disease appeared again in 2007 when 220 cases of the infection had been registered. While the disease spread in 2008, with 360 confirmed cases, the pandemic became
rampant in 2009 to the degree that Taizi people have become terrified. The number of infected cases has reached 906.

[Although there are wide differences in the numbers of reported dengue cases and fatalities cited in the report above, it is clear that a major dengue outbreak has been occurring in Taiz. Descriptions of a number of anecdotal dengue cases, including fatalities, mentioned in this report were not included.

BACKGROUND: Rio de Janeiro, Brazil, experienced a severe dengue fever epidemic in 2008. This was the worst epidemic ever, characterized by a sharp increase in case-fatality rate, mainly among younger individuals. A combination of factors, such as climate, mosquito abundance, buildup of the susceptible population, or viral evolution, could explain the severity of this epidemic. The main objective of this study is to model the spatial patterns of dengue seroprevalence in three neighborhoods with different socioeconomic profiles in Rio de Janeiro. As blood sampling coincided with the peak of dengue transmission, we were also able to identify recent dengue infections and visually relate them to Aedes aegypti spatial distribution abundance. We analyzed individual and spatial factors associated with seroprevalence using Generalized Additive Model (GAM).

METHODOLOGY/PRINCIPAL FINDINGS: Three neighborhoods were investigated: a central urban neighborhood, and two isolated areas characterized as a slum and a suburban area. Weekly mosquito collections started in September 2006 and continued until March 2008. In each study area, 40 adult traps and 40 egg traps were installed in a random sample of premises, and two infestation indexes calculated: mean adult density and mean egg density. Sera from individuals living in the three neighborhoods were collected before the 2008 epidemic (July through November 2007) and during the epidemic (February through April 2008). Sera were tested for DENV-reactive IgM, IgG, Nested RT-PCR, and Real Time RT-PCR. From the before-after epidemics paired data, we described seroprevalence, recent dengue infections (asymptomatic or not), and seroconversion. Recent dengue infection varied from 1.3% to 14.1% among study areas. The highest IgM seropositivity occurred in the slum, where mosquito abundance was the lowest, but household conditions were the best for promoting contact between hosts and vectors. By fitting spatial GAM we found dengue seroprevalence hotspots located at the entrances of the two isolated communities, which are commercial activity areas with high human movement. No association between recent dengue infection and household’s high mosquito abundance was observed in this sample.

CONCLUSIONS/SIGNIFICANCE: This study contributes to better understanding the dynamics of dengue in Rio de Janeiro by assessing the relationship between dengue seroprevalence, recent dengue infection, and vector density. In conclusion, the variation in spatial seroprevalence patterns inside the neighborhoods, with significantly higher risk patches close to the areas with large human movement, suggests that humans may be responsible for virus inflow to small neighborhoods in Rio de Janeiro. Surveillance guidelines should be further discussed, considering these findings, particularly the spatial patterns for both human and mosquito populations.

SANAA, 16 October 2009 (IRIN) – Hundreds of people in Taiz city, 250km south of the Yemeni capital, Sanaa, have dengue fever and local hospitals are taking in new cases on a daily basis, according to health officials.

“Since the new outbreak in September, at least 350 cases have been confirmed and a further 1,000 are suspected,” Mohammed Mahmoud, manager of the government’s National Malaria Control Programme (NMCP) in Taiz, told IRIN.

He said the number of infections had increased due to the widespread use of uncovered water tanks, particularly in the city’s slums. “Swamps and open sewers contribute much to the reproduction of the vector, known as Aedes [mosquito] that transmits the fever,” Mahmoud said.

Many cases go undiagnosed and are not properly treated as most of those infected do not go to hospital for screening, said Huwaida al-Shathili, a professor at Taiz university’s faculty of medicine. Al-Shathili said the number of dengue-infected cases may be many times more than those registered by NMCP, as symptoms can be difficult to detect.

Doctors say Taiz (population 500,000) has a fertile environment for mosquito breeding. “In Taiz, dengue isn’t an epidemic… it is a recurring disease in the governorate where stagnant water and pollution are commonplace… The disease appears every two or three years,” al-Shathili noted. “Last year, more than 300 cases were detected.”

Epidemiology

According to the World Health Organization (WHO), dengue is transmitted by the bite of an Aedes mosquito infected with any one of the four dengue viruses. It occurs in tropical and sub-tropical areas. Symptoms appear 3-14 days after a bite from an infected mosquito.

Common dengue symptoms include high fever, vomiting, headache, acute pain in the joints and skin rash, according to Murshid Hassan, a senior health official in the governorate. “Infected cases must have access to immediate treatment under the supervision of physicians,” Hassan said. A WHO factsheet states: “There is no specific treatment for dengue, but appropriate medical care frequently saves the lives of patients with the more serious dengue haemorrhagic fever.”

Spraying campaigns were undertaken in some parts of the city in September but results have been poor due to a lack of funding, according to Mahmoud.

“We proposed two massive spraying campaigns to cover all 94,000 houses in the city at a cost of YR 38 million [US$190,000],” he said, adding that the Health Ministry had provided insufficient funding.

HYDERABAD: While slum belts in the Hyderabad district have been identified as likely to be the worst-hit in the case of heavy rainfall, besides the problems associated with water logging, a bevy of diseases are likely to to develop in the areas, unless necessary administrative checks are in place.

With the swine flu scare adding to the concerns of the health departments in the district, the possibility of epidemic outbreaks are keeping health officials on their toes.

In this connection, the District Medical Health authorities revealed on Sunday that while weekly meetings have been initiated with HMWSSB officials for monitoring sanitation issues, a number of additional measures are also being instituted.

Officials indicated that specific problem- prone areas in Hyderabad district have been identified, and an action plan is being formulated in this regard.

District Medical Health Officer for the Hyderabad district, Ch Jayakumari said on Sunday that the possibility of dengue fever is on the rise in the slums and low lying areas in the Twin Cities.

While officials have reportedly started initiating sanitation drives in low lying areas with bleaching powder and similar disinfectant substances as a deterrent for mosquito larvae, the GHMC officials meanwhile have been given the responsibility of initiating local level drives to alleviate the situation in areas prone to water stagnation. Sources indicated that measures regarding application of bleaching powder may not be adequate to cover all the areas at the local level in the district, and as such, individuals should exercise discretion to sanitise their locality on their own accord.

Jayakumari noted that a team from the Integrated Disease Surveillance Project (IDSP) along with the District Epidemic Cell is presently monitoring the situation in the district, with special regards to waterborne diseases. The central surveillance unit will coordinate the activities of the IDSP at the national level providing technical support, and guidance along with financial support. A number of medical and paramedical staff in Government hospitals have also been deployed to monitor the disease situation in the high risk areas.

Officials said that two mobile teams have been dispatched from the DHMO office, who along with officials from the GHMC have be instructed to keep a watch on individual areas. Mobile Treatment Centres (MTCs) are also being reportedly readied to tackle the likelihood of an outbreak, with special emphasis on schools and slum areas in the district.

Meanwhile, information regarding the administration of oral rehydrating solution (ORS) in the case of disease management is being conveyed at the local area level, in conjunction with Asha workers and medical officers.

The areas that have been identified so far include Aman Nagar, Azampura, Charminar, Barkas, Kishan Bag, Bholakpur, Asifnagar, Borabanda, Viveknagar, Agapura, Ramnagar, Mettuguda, Gudimalkapur among others in the district, informed the Health Officer.